This invention relates generally to multichannel record disc reproducing systems. More particularly, it relates to a system for demodulating and reproducing without distortion and with a high signal-to-noise ratio. The reproduced signal comprises an angle-modulated signal picked up from a multichannel record disc through the use of a plurality of phase-locked loops having, respectively, different lock ranges.
Systems for demodulating angle-modulated signals by means of phase-locked loops (hereinafter referred to as PLL) have been known in the prior art. A PLL of this type generally comprises a phase comparator which is supplied with an angle-modulated signal as one input. A loop filter is used for attenuating the high-frequency component of an output error signal from the phase comparator. A direct-current amplifier amplifies the output signals from the loop filter. A voltage-controlled oscillator generates a frequency which is controlled by the output of the direct-current amplifier. The resulting output of the oscillator is supplied, as a phase comparison signal, to the phase comparator. A demodulated output signal is derived from the direct-current amplifier. The direct-current amplifier may be omitted. The frequency characteristic of the demodulated signal is determined by the width of the lock range or capture range of the PLL.
There may be a carrier drop in the signal reproduced from a multichannel record disc, on which a direct-wave signal and an angle-modulated signal have been multiplexed and recorded. That is, an absence of the carrier component may occur in the reproduced angle-modulated signal or there may be an over-modulation due to a disturbance caused by a frequency close to the carrier frequency. This close frequency may be a higher harmonic of the direct wave. As a result, an abnormal noise is generated.
Accordingly, in order to reduce or eliminate this noise, it has been a conventional practice to constrict the lock range of the PLL when an abnormal noise occurs. By this expedient, an abnormal locking of the PLL is prevented. Furthermore, the high-frequency component (for example, in the vicinity of 8 KHz) of the demodulated signal is attenuated, whereby the apparent noise level is lowered. As is known, the lock range or capture range of a PLL is determined by the loop gain of the PLL. This loop gain is determined principally by quantities such such factors as the gain of the direct-current amplifier, the passing quantity of the loop filter, and the conversion gain of the phase comparator and the voltage-controlled oscillator. The prior art has varied the lock range by increasing and decreasing this loop gain of the PLL, so that the characteristic of the loop filter is varied.
Usually, this loop filter includes a lag-lead filter comprising a fixed resistor connected between the input and output terminals and a series connection of a capacitor and a variable resistor connected between ground and the junction between the fixed resistor and the output terminal. This combination constitutes a kind of low-pass filter. By varying the resistance of the variable resistor in this loop filter, the characteristic of this loop filter is changed, and the loop gain of the PLL varies. Hence, the lock range of the loop is appropriately varied and set. Here, increasing the resistance value of the variable resistor of the loop filter causes the lock range width to increase, while decreasing the same resistance value causes the lock range width to decrease.
An angle-modulated signal is obtained by modulating a carrier with a modulation signal in which much energy is distributed particularly in a high-frequency range (in the vicinity of 8 KHz). For example, this is the frequency range which may include the sound of cymbals. When such a signal is reproduced and is introduced into the PLL, its lock range is made narrower than the deflection frequency width of the angle-modulated signal. This narrowing is accomplished by varying the above mentioned variable resistor. As a result, even when there are higher harmonics of the direct-wave signal, admixed in the angle-modulated wave band, the PLL does not erroneously lock with these higher harmonics. Furthermore, as a result of the smaller lock range of the PLL, there is an attenuation of the high-frequency band of the demodulation by the PLL. Therefore, even if the PLL should unlock, the noise in the high-frequency band will be attenuated, and a great noise will not be sensed.
An angle-modulated signal may be obtained by modulating a carrier with a modulation signal, in which much energy is distributed particularly in a low-frequency range (in the vicinity of 2 KHz). For example, a signal such as this may include the sound of a trumpet. If the deviation frequency of this angle-modulated signal exceeds the lock range, this angle-modulated signal is relatively overmodulated with respect to the lock range of the PLL. In this case, an abnormal noise is generated in the output signal of the PLL despite the angle-modulated wave. This abnormal noise is very unpleasant to the ear.
Accordingly, the lock range of the PLL is made uniformly narrow in a conventional demodulation system in which a PLL is used. Therefore, the abnormal noise can be neglected from the standpoint of the sense hearing an angle-modulated signal produced by modulation a high frequency. However, if an angle-modulated signal is produced by modulation of a medium low frequency, the PLL unlocks, despite the normal angle-modulated signal. Beat sounds are generated by the input signal of the voltage-controlled oscillator and the input of the PLL. These beat sounds are very unpleasant to the ear.
Conversely, if the lock range of the PLL is broadened, the PLL responds rapidly. Although unlocking will not occur, the PLL responds to even a slight abnormality. Furthermore, the demodulation characteristic of the PLL extends up to the high-frequency range, at which it does not attenuate very much. For this reason, an unpleasant abnormal noise is heard.
Thus, there are mutually different advantages and disadvantages in the case of a narrow lock range and a wide lock range of the PLL. Consequently, it has not been possible to demodulate completely, without distortion or generation of noise, in a system wherein a single PLL is used for an input angle-modulated signal, and its lock range is controlled, as in the prior art.